A Long Term Evolution (LTE) cellular radio communication system uses Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink (evolved NodeBs to user equipments) and Discrete Fourier Transform (DFT)-spread OFDM in the uplink (user equipments to evolved NodeBs). The LTE system is an evolution of the widely deployed wideband code division multiple access (WCDMA) systems and is standardized by the Third Generation Partnership Project (3GPP) in Technical Specifications (TS)Series 36, generally Release 8 (Rel-8), and later Releases. An LTE system is sometimes also called the Evolved Universal Terrestrial Radio Access (E-UTRA) communication system.
The basic LTE physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier and bandwidth of 15 kilohertz (kHz) during one OFDM symbol interval. Each OFDM symbol can have either a normal (short) or extended (long) cyclic prefix. In the time domain, LTE transmissions are organized into successive radio frames of length Tframe=10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as depicted by FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of physical resource blocks (PRBs or RBs), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 subcarriers, or tones, in the frequency domain. Resource blocks are numbered in the frequency domain, starting from 0 at one end of the system bandwidth. Each slot includes either six or seven OFDM symbols, depending on the length of the symbols' cyclic prefixes.
Uplink (UL) transmissions are dynamically scheduled, i.e., a Physical Downlink Control CHannel (PDCCH) in each subframe indicates if resources on a Physical Uplink Shared CHannel (PUSCH) are granted to a terminal, or user equipment (UE), for an UL transmission. If so, the resources granted are valid for subframe n+4 if the uplink grant has been transmitted in subframe n. This simple relation is valid for a Frequency Division Duplex (FDD) mode of operation; in a Time Division Duplex (TDD) mode, the fact that not all subframes are UL subframes has to be taken into account.
LTE uses hybrid Automatic Repeat Request (hybrid ARQ, or HARQ), where after receiving uplink data in a subframe, the evolved NodeB (eNB) attempts to decode it and reports to the terminal whether the decoding was successful (acknowledgement, or ACK) or not (non-acknowledgement, or NACK). In case of an unsuccessful decoding attempt, the terminal can retransmit the erroneous data.
PHICH:
The ACK or NACK indication is transmitted by an evolved NodeB (eNB) on a Physical Hybrid ARQ Indicator Channel (PHICH). For FDD mode, the PHICH is transmitted in subframe n+8 if uplink resources have been granted in subframe n and the UL transmission has been performed in subframe n+4. The overall transmission bandwidth is shared among terminals, i.e., within one subframe, multiple terminals can get uplink resources granted. In LTE, the primary multiple access scheme in the uplink is Frequency Division Multiple Access (FDMA), and therefore the LTE uplink transmission scheme is also referred to as Single Carrier-FDMA to account for both the single carrier property of DFT-spread OFDM as well as the selected multiple access system.
Besides FDMA, LTE can also provide Spatial Domain Multiple Access (SDMA), where multiple UEs are granted the same time-frequency resources, but for SDMA to work, the wireless channels of the terminals scheduled on the same resources must possess special properties. Furthermore, each UE must use orthogonal uplink DeModulation Reference Signals (DMRS), which are generated by cyclic shifting a common base sequence by different amounts. In the context of LTE, uplink SDMA is typically referred to as uplink Multi User Multiple Input Multiple Output (MU-MIMO).
If multiple terminals perform multiple PUSCH transmissions in the same subframe, the corresponding HARQ feedback transmissions occur in the same subframe four subframes later (in FDD). To avoid collisions, the resources on the PHICH are derived by the eNB from a first physical resource block index IPRB—RAlowest—index of the PUSCH transmissions (which enables unique PHICH resources for FDMA) and the cyclic shift (CS) nDMRS of the uplink DMRS (unique resources for uplink MU-MIMO).
For dynamically scheduled transmissions, the eNB uses the following Eq. 1 to calculate a PHICH group nPHICHgroup and PHICH sequence nPHICHseq, as specified in, for example, Clause 9.1.2 of 3GPP TS 36.213 V8.8.0, Physical layer procedures (Release 8) (September 2009). The PHICH group and sequence together determine a unique PHICH resource.nPHICHgroup=(IPRB13RAlowest—index+nDMRS)mod NPHICHgroup+IPHICHNPHICHgroup nPHICHseq=(└IPRB—RAlowest—index/NPHICHgroup┘+nDMRS)mod 2NSFPHICH  (1)in which └x┘ indicates the floor function of x, i.e., the largest integer less than or equal to x, and the parameter NPHICHgroup is the number of available PHICH groups, which can be derived from higher layer parameters, as described for example in Clause 6.9 of 3GPP TS 36.211 V9.1.0, Physical Channels and Modulation, Release 9 (December 2009). The parameter NSFPHICH is the spreading factor (SF) size used for PHICH (e.g., 8 and 4 for normal and extended cyclic prefix, respectively), as specified in, for example, Clause 6.9.1 of 3GPP TS 36.211. The parameter IPHICH is 0 for FDD and 1 for some TDD transmissions, as described in Clause 9.1.2 of 3GPP TS 36.213.Carrier Aggregation:
The LTE Rel-8 standard supports bandwidths up to 20 megahertz (MHz), but 3GPP has initiated work on LTE Release 10 (Rel-10) to meet the requirements of an IMT-Advanced system (i.e., a “fourth generation” (4G) system that uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, high speed packet access (HSPA), or other communication system for IMS multimedia telephony (IMT)). One of the parts of LTE Rel-10 is to support bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8, which includes spectrum compatibility. Thus, an LTE Rel-10 carrier that is wider than 20 MHz should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In LTE Rel-10, CCs are also called (serving) cells, in particular Primary and Secondary cells.
In particular for early LTE Rel-10 deployments, it can be expected that there will be a small number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide Rel-10 carrier also for legacy (Rel-8) terminals, i.e., it is possible to implement carriers where legacy terminals can be scheduled in all parts of a wideband LTE Rel-10 carrier. A straightforward way to obtain this would be by Carrier Aggregation (CA), in which an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 3, which depicts five CCs, each having a bandwidth of 20 MHz, that are aggregated to a combined bandwidth of 100 MHz.
The number of aggregated CCs as well as the bandwidths of the individual CCs may be different for uplink and downlink. In a symmetric configuration, the number of CCs in downlink is the same as the number of CCs in uplink, and in an asymmetric configuration, the numbers of CCs in the downlink and uplink are different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. Thus, a terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
Depending on the exact configuration, uplink grants transmitted by an eNB on the PDCCH of one downlink CC can schedule multiple uplink CCs or just a single uplink CC.
FIG. 4 shows the case that PDCCH on one downlink CC schedules PUSCH transmissions on only a single uplink CC (RB=IPRB—RAlowest—index, CS=nDMRS). In particular in FIG. 4, an UL grant 1 for RB1, CS1 transmitted on the PDCCH on a DL CC 200 prompts a PUSCH transmission, according to the received grant, on an UL CC 202 that is acknowledged by an ACK/NACK 1 transmission on the PHICH on the DL CC 200. Whether an ACK or a NACK is transmitted depends on the received RB1, CS1 PUSCH transmission. In a similar way, an UL grant 2 for RB2, CS2 transmitted on another DL CC 204 prompts a PUSCH transmission on an UL CC 206 that is acknowledged by an ACK/NACK 2 transmission on DL CC 204.
FIG. 5 shows the case that the PUSCH transmissions on multiple UL CCs can be scheduled from a PDCCH on a single downlink CC (RB=IPRB—RAlowest—index, CS=nDMRS). In particular in FIG. 5, an UL grant 1 for RB1, CS1 transmitted on a DL CC 210 prompts a PUSCH transmission on an UL CC 212 that is acknowledged by an ACK/NACK 1 transmission on the DL CC 210, and an UL grant 2 for RB2, CS2 transmitted on the DL CC 210 prompts a PUSCH transmission on an UL CC 214 that is acknowledged by an ACK/NACK 2 transmission on the DL CC 210.
The PHICH of a PUSCH transmission takes place on the same CC that has been used to transmit the uplink grant. In the case shown in FIG. 4, no PHICH collisions can occur. In the case shown in FIG. 5, PHICH collisions can occur. Assuming that in FIG. 5 PUSCH transmissions on both CCs start with the same first uplink resource block IPRB—RAlowest—index and use the same cyclic shift nDMRS for the uplink DMRS, the same PHICH resources are going to be used according to Eq. (1), resulting in a PHICH collision.
To mitigate this problem, an offset can be added to the resource block numbering of a CC. A special case thereof is to number the resource blocks contiguously across CCs, i.e., on the first UL CC, RBs are numbered from 0 to NRBUL,CC1−1; on the second CC, RBs are numbered from NRBUL,CC1 to NRBUL,CC1+NRBUL,CC2−1; etc. Nevertheless, contiguous numbering can be seen as a special case of the more general idea to offset the resource block numbering on each CC by a CC-specific offset osCCn, i.e., to use IPRB—RAlowest—index+osCCn instead of IPRB—RAlowest—index in Eq. (1) to calculate PHICH resources for PUSCH transmission on CC n. With this transformation, the PHICH resource assignment formula of Eq. (1) effectively becomes:nPHICHgroup=(IPRB—RAlowest—index+osCCn+nDMRS)mod NPHICHgroup+IPHICHNPHICHgroup nPHICHseq=(└(IPRB—RAlowest—index+osCCn)/NPHICHgroup┘+nDMRS)mod 2NSFPHICH  (2)for CC n.
The solution outlined by Eq. (2) may not be necessary for dynamically scheduled transmissions since there the cyclic shift nDMRS of uplink DMRS can be used to generate unique PHICH resources for different CCs. However, for semi-persistent scheduled transmissions, nDMRS is always set to 0, requiring a modification to the PHICH resource calculation, with Eq. (2) being one example.
While Eq. (2) works to avoid PHICH collisions, it complicates scheduling since it modifies both the PHICH group nPHICHgroup as well as the PHICH sequence nPHICHseq. A modification of nPHICHgroup is however not needed since all possible nPHICHgroup can be reached even with the unmodified resource block numbering from 0 to NRBUL,CCn−1. For example, looking at FDD (where IPHICH is always zero) with normal cyclic prefix, the maximum number of PHICH groups is ceil(NRBDL/4) according to 3GPP TS 36.211. Ceil(x) is the ceiling function of x, i.e., the smallest integer greater than or equal to x. Assuming for simplicity NRBUL,CCn=NRBDL (i.e., the same uplink and downlink bandwidth for CC n), each PHICH group is visited four times when varying the resource block index IPRB—RAlowest—index from 0 to NRBUL,CCn−1 in Eq. (1).
The problem however is the PHICH sequence number nPHICHseq. Assuming again FDD with normal cyclic prefix and semi-persistent scheduling (i.e., nDMRS=0), we see from Eq. (1) that the highest possible PHICH sequence number is nPHICHseq=floor((NRBUL−1)/ceil(NRBUL/4))mod 2NSFPHICH=3, in which floor(x) is the above-described floor function of x. Thus, the higher PHICH sequences 4 to 7 (normal cyclic prefix) can never be reached.